﻿<p>The <em>IfcMaterialLayerSetUsage</em> determines the usage of <em>IfcMaterialLayerSet</em> in terms of its location and
orientation relative to the associated element geometry. The location of material layer set shall be compatible with the
building element geometry (that is, material layers shall fit inside the element geometry). The rules to ensure the compatibility 
depend on the type of the building element.</p>

<blockquote class="example">EXAMPLE&nbsp; For a cavity brick wall with shape representation SweptSolid, the <em>IfcMaterialLayerSet.TotalThickness</em> 
shall be equal to the wall thickness. Also the <em>OffsetFromReferenceLine</em> shall match the exact positions between the two shape representations
of <em>IfcWallStandardCase</em>, that is the <em>IfcShapeRepresentation</em>'s with <em>RepresentationIdentifier</em>="Axis" and
<em>RepresentationIdentifier</em>="Body".</blockquote>

<blockquote class="note">NOTE&nbsp; Model view definitions or implementer agreements may provide more instructions on matching between building element geometry and material layer set usage.</blockquote>

<p>The <em>IfcMaterialLayerSetUsage</em> is always assigned to an individual occurrence object (and only to relevant subtypes of
<em>IfcElement</em>). The <em>IfcMaterialLayerSet</em>, referenced by <em>ForLayerSet</em>, can however be shared among several occurrence
objects. If the element type is available (in other words, an instance of the relevant subtype of <em>IfcElementType</em> exists), then the
<em>IfcMaterialLayerSet</em> can be assigned to the element type. The assignment between a subtype of <em>IfcElement</em> and the
<em>IfcMaterialLayerSetUsage</em> is handled by <em>IfcRelAssociatesMaterial</em>.</p>

<p class="use-head">Attribute use definition</p>
<p>The <em>IfcMaterialLayerSetUsage</em> is primarily intended to be associated with planar building elements having a constant
thickness. With further agreements on the interpretation of <em>LayerSetDirection</em>, the usage can be extended also to other
cases, for example to curved building elements, provided that the material layer thicknesses are constant.</p>

<p>Generally, an element may be layered in any of its primary directions, denoted by its x, y or z axis. The geometry use
definitions at each specific type of building element will determine the applicable <em>LayerSetDirection</em>.</p>

<p>The following examples illustrate how the <em>IfcMaterialLayerSetUsage</em> attributes (<em>LayerSetDirection</em>, <em>DirectionSense</em>, 
<em>OffsetFromReferenceLine</em>) can be used in different cases. Normative material use definitions are documented at each element 
(how these shall be used).</p>

<p>Figure 4 shows an example of the use of <em>IfcMaterialLayerSetUsage</em> aligned to the axis of a wall.</p>

<blockquote class="example">EXAMPLE&nbsp; For a standard wall with extruded geometric representation (vertical extrusion), the layer set
direction will be perpendicular to extrusion direction, and can be derived from the direction of the wall axis. With the<em>DirectionSense</em> (positive in this example) the individual <em>IfcMaterialLayers</em> are assigned consecutively right-to-left or left-to-right. For a curved wall, 
"direction denoting the wall thickness" can be derived from the direction of the wall axis, and it will remain perpendicular to the wall path. 
The <em>DirectionSense</em> applies as well.</blockquote>

<blockquote class="note">NOTE&nbsp; According to the <em>IfcWallStandardCase</em> material use definition the <em>LayerSetDirection</em> for
<em>IfcWallStandardCase</em> is always AXIS2 (that is, along the y-axis), as shown in this example.</small></blockquote>

<table>
<tr><td><img src="../../../figures/ifcmateriallayersetusage_wall-01.png" alt="Mls usage"border="0"></td></tr>
<tr><td><p class="figure">Figure 4 &mdash; Material layer set usage for wall</p></td></tr>
</table>

<p>Figure 5 shows an example of the use of <em>IfcMaterialLayerSetUsage</em> aligned to a slab.</p>

<blockquote class="example">EXAMPLE&nbsp; For a slab with perpendicular extruded geometric representation, the <em>LayerSetDirection</em>
will coincide with the extrusion direction (in positive or negative sense). In this example, the material layer set base is
the extruded profile and consistent with the <em>IfcExtrudedAreaSolid.Position</em>, with the <em>DirectionSense</em> being positive, the
individual <em>IfcMaterialLayers</em> are built up from the base towards the positive z direction in this case.</blockquote>

<blockquote class="note">NOTE&nbsp; According to the <em>IfcSlabStandardCase</em> material use definition the <em>LayerSetDirection</em> for
<em>IfcSlabStandardCase</em> is always AXIS3 (that is, along the z-axis).</blockquote>

<table>
<tr><td><img src="../../../figures/ifcmateriallayersetusage_slab-01.png" alt="Mls usage" border="0" height="320" width="601"></td></tr>
<tr><td><p class="figure">Figure 5 &mdash; Material layer set usage for slab</p></td></tr>
</table>

<p>Figure 6 shows an example of the use of <em>IfcMaterialLayerSetUsage</em> aligned to a roof slab with non-perpendicular extrusion.</p>

<blockquote class="example">EXAMPLE&nbsp; For a slab with non-perpendicular extruded geometric representation, the guidelines above apply as
well</small><small>. The material layer thickness and the <em>OffsetFromReferenceLine</em> are always measured
perpendicularly, even if the extrusion direction is not perpendicular. Therefore the total material layer thickness is
not equal to the extrusion depth of the geometry.</blockquote>

<table>
<tr><td><img src="../../../figures/ifcmateriallayersetusage_roofslab-01.png" alt="Mls usage" border="0"></td></tr>
<tr><td><p class="figure">Figure 6 &mdash; Material layer set usage for roof slab</p></td></tr>
</table>